Tantalum-tungsten alloy

ABSTRACT

A NOVEL ALLOY IS DISCLOSED CONSISTING ESSENTIALLY OF FROM 1.5 TO 3.5 WEIGHT PERCENT OF TUNGSTEN, THE BALANCE OF THE ALLOY BEING ESSENTIALLY TANTALUM, OPTIONALLY, THE ALLOY CONTAINS FROM 0.05 TO 0.5 WEIGHT OF COLUMBIUM. THESE ALLOYS HAVE IMPROVED STRENGTH WHEN COMPARED WITH PURE TANTALUM, YET THEY ARE COLD WORKABLE IN CONVENTIONAL EQUIPMENT FOR COLD-WORKING TANALUM, UNLIKE THE KNOWN TANTALUM ALLOYS HAVING HIGHER CONCENTRATIONS OF TUNGSTEN. ALSO, THE ALLOYS DISCLOSED HEREIN SHOW IMPROVED CORROSION RESISTANCE OVER PURE TANTALUM AND THE KNOWN TANTALUM-TUNGSTEN ALLOYS HAVING A HIGHER TUNGSTEN CONCENTRATION .

United States Patent 3,592,639 TANTALUM-TUNGSTEN ALLOY MortimerSchussler, Joppa, and Victor T. Bates, Millersville, Md., assignors toFansteel Inc. No Drawing. Filed Aug. 19, 1968, Ser. No. 753,721 Int. Cl.C22c 27/00 US. Cl. 75-174 5 Claims ABSTRACT OF THE DISCLOSURE A novelalloy is disclosed consisting essentially of from 1.5 to 3.5 weightpercent of tungsten, the balance of the alloy being essentiallytantalum. Optionally, the alloy contains from 0.05 to 0.5 weight percentof columbium. These alloys have improved strength when compared withpure tantalum, yet they are cold workable in conventional equipment forcold-working tantalum, unlike the known tantalum-tungsten alloys havinghigher concentrations of tungsten. Also, the alloys disclosed hereinshow improved corrosion resistance over pure tantalum and the knowntantalum-tungsten alloys having a higher tungsten concentration.

BACKGROUND OF THE INVENTION 7 Tantalum and its alloys are commonly usedin environments in which their superior corrosion resistance can berelied upon to provide long lived components such as bayonet heaters andheat exchangers exposed to corrosive solutions such as sulfuric acid,hydrochloric acid, and nitric acid.

Pure tantalum is often cold worked, i.e. at temperatures below the point(about 1000 F.) where significant oxidation can occur on the tantalum inthe presence of air. This cold working generally is performed bymachinery such as a 4,000 pound steam hammer, to prepare shaped tantalumarticles for commercial use.

In some applications in which the tantalum is required to endure highstress or pressure at elevated temperatures of about 400 F. or above,the strength properties of pure tantalum are marginal, and a strongermaterial is frequently needed, such as in chemical process equipment,especially in the forms of tubing and sheet.

It is known to add tungsten, eg about 5 to 10 weight percent, totantalum is produce alloys having a higher tensile strength than puretantalum, but large pieces of such alloys (e.g. having dimensionsgreater than 2 or 3 inches in thickness) are too hard to be cold workedon the commercial equipment which is generally used to cold work puretantalum. Thus, when a tantalum alloy of higher strength than puretantalum is needed to fabricate a part of larger size, manufacturershave been previously forced to either hot work tantalum-tungsten alloysin an air atmosphere, which causes degradation in the materials becauseof embrittlement of the surface layers due to diffusion of oxygen andnitrogen into the metal, or to coldwork the alloys in extremely heavyand expensive equipment.

This application relates to tantalum-tungsten alloys havingsubstantially higher tensile strength than pure tantalum, but which arecold workable in equipment con-' ventionally used to cold work puretantalum. Furthermore, the alloys of this invention exhibit improvedcorrosion resistance to acids when compared with either pure tantanlumor the known tantalum-tungsten alloys of high-" er tungstenconcentration.

SUMMARY OF THE INVENTION This application relates to a single phase,cold workable alloy having high corrosion resistance and consisting "iceessentially of from 1.5 to 3.5 weight percent of tungsten, the balanceof said alloy being essentially tantalum.

Preferably, the above alloy also contains from about 0.05 to 0.5 Weightpercent of columbium (also known as niobium). It is believed that thepresence of such a small amount of columbium aids in reduction of thegrain size in the alloy, resulting in a material having better physicalproperties.

The capability of an alloy to be cold worked can be looked upon as afunction of the difference between its yield strength '(the amount ofstress required to make a bar of the alloy permanently deform or stretcha distance of 0.2 percent of the length of the bar) and its ultimatetensile strength (the amount of stress required to break the bar ofalloy). By application of stresses having a magnitude ranging betweenthe value of these two tensile strengths, it is possible to form orshape a piece of metal into a desired configuration Without breaking it.

The alloys of this invention typically have a yield tensile strength atambient temperatures of at least about 30,000 p.s.i. and an ultimatetensile strength at ambient temperatures of at least about 46,000 p.s.i.The wide gap of about 16,000 p.s.i. between the two tensile strengthsindicates that the alloys of this invention are readily cold workable byrolling or forging into strong metal sheeting, metal cups, or otherstructures as desired.

At ambient temperatures, the yield tensile strength of an alloy of thisinvention containing 2.5 weight percent of tungsten is about 35,000p.s.i. while the ultimate tensile strength is about 53,000 p.s.i. Thedilference between these two values is about 18,000 p.s.i., which isabout 35 percent of the value of the ultimate tensile strength.

By comparison, a tantalum-tungsten alloy containing 10 weight percent oftungsten has, at ambient temperatures, a yield tensile strength of about89,000 p.s.i. and an ultimate tensile strength of about 96,000 p.s.i.This material is too hard for large pieces to be cold worked inconventional equipment for Working pure tantalum. Also, the differencebetween these two tensile strengths is only about 7,000 p.s.i., or lessthan 10 percent of the ultimate tensile strength. This compares with the16,000 p.s.i. difference between the two tensile strengths of the alloysof this invention.

The above principles also hold true at temperatures up to the maximumcold working temperatures. For example, at 600 C. (1040 R), an alloy ofthis invention containing 2.5 weight percent of tungsten has a yieldtensile strength of about 15,000 p.s.i. and an ultimate tensile strengthof about 34,000 p.s.i. The difference between these two tensilestrengths is about 19,000 p.s.i., more than one half the value of theultimate tensile strength at that temperature. Thus it can be seen thatthe gap between yield strength and ultimate tensile strength does notdiminish, although both strength values naturally drop on heating.

A wide spread between the yield tensile strength and ultimate tensilestrength of a metal is particularly important because a piece of metalbeing worked is subjected to different magnitudes of stress at differentportions of the metal. In order for a metal object to be successfullyshaped each portion of the object must be subjected to stresses whichare above the yield tensile strength so that each portion of the metalobject can be deformed, but the stresses on each portion must be belowthe ulti mate tensile strength so that cracks do not form. Since themagnitude of the stress forces can vary considerably throughout thevarious portions of the metal object to be shaped, a wide range betweenthe yield and ultimate tensile strength is important.

When the prior tantalum-tungsten alloys of higher .tungsten content areheated, their tensile strength also drops, but not to such a degree thatthey can be cold worked in conventional machinery for working tantalum.Also, the diiferences between their yield tensile strengths and ultimatetensile strengths are much lower at high temperatures than in the alloysof this invention. This causes such alloys to be less ductile than thealloys of this invention.

Alloys of this invention which have optimum cold working characteristicsand which show optimum corrosion resistance to acids are obtained whenthe tungsten content is from about 2 to 3 weight percent. These alloyshave yield tensile strengths ranging from about 32,000 to 38,000 p.s.i.and ultimate tensile strengths from about 48,000 to 55,000 p.s.i., atambient temperatures. They combine the advantages of substantiallygreater strength than pure tantalum, which has a yield tensile strengthof about 24,000 p.s.i. and an ultimate tensile strength of about 37,000p.s.i. at ambient temperatures, with the capability of being cold workedon the conventional machinery used for cold working pure tantalum. Inaddition to this, and surprisingly, the corrosion resistance of thesealloys exceeds that of pure tantalum and any other knowntantalum-tungsten alloy.

DESCRIPTION OF SPECIFIC EMBODIMENT The following example illustrates atypical alloy of this invention having the above desirablecharacteristics, and should not be construed as limiting this invention,which is defined in the claims.

A metal ingot having the composition described below was produced byconventionally electron beam melting blends of tantalum, tungsten, andcolumbium powders. The ingot was re-melted several times by an electronbeam to achieve an essentially homogeneous alloy of one phase.

After melting, the alloy consisted primarily of tantalum (about 97.74weight percent), but contained other ingredients as shown below in thefollowing proportions, taken as an average of the values obtained fromtwo analyses at the two ends of the ingot:

tungsten2.5 wt. percent columbium-0.08 wt. percent carbonless thanp.p.mfi" nitrogenless than p.p.m. oxygen33 p.p.m. hydrogenless than 5p.p.m. molybdenumless than 5 p.p.m. coba1tless than 5 p.p.m. iron10p.p.m. vanadiumless than 5 p.p.m. titanium-less than 5 p.p.m.zirconiumless than 5 p.p.m. siliconless than 5 p.p.m. magnesium-lessthan 5 p.p.m. tinless than 5 p.p.m. copper-less than 5 p.p.m. nickellessthan 5 p.p.m. aluminumless than 5 p.p.m. calcium--less than 5 p.p.m.manganeseless than 5 p.p.m.

chromium-10 p.p.m.

* Parts per million.

The above metal ingot, having the shape of a solid cylindrical rod seveninches in diameter, was forged on a 4,000 lb. steam hammer into a longrectangular bar six inches wide and two inches high. The temperature ofthe ingot during forging was about 900 R, which is below the temperatureat which significant oxidation of the ingot takes place in air.

A cut portion of the bar of alloy was then repeatedly cold rolled, withan intermediate anneal at about 1400 C. (2550 F.) for 60 minutes, to ainch plate. Analysis of the cut portion of the bar used showed it tocontain about 2.7 weight percent of tungsten and 0.16 weight 4 percentof columbium, the balance being essentially tantalum.

The inch plate of alloy was then annealed again at 1250 C. 2280 F.) for60 minutes, and rolled to a thickness of 0.02 inch. Following this, itwas annealed at 1200 C. (2190 F.) for 60 minutes to achieve 100 percentrecrystallization. The grains were of small size (ASTM grain size No. 7/2) compared with a similarly processed sheet of pure tantalum which hada larger grain size (ASTM grain size No. 4 /2 or 5).

The rolled sheet of alloy showed excellent bend ductility andweldability. Welded and re-drawn tubing can be processed from the alloysheet without difliculty.

The recrystallized 0.02 inch sheet of alloy was tested for its yieldtensile strength and its ultimate tensile strength (using those terms asdefined above) at 200 C. (392 F.). The elongation of the sheet at itsbreak point was also determined as a percentage of the length of thesheet in a 2-inch gage length. As a control, a 0.02 inch thick sheet ofrecrystallized tantalum was similarly tested. The results were:

Ultimate 1 0.2 percent ofiset, p.s.i. 2 Percent in Z-ineh gage length.

At ambient temperatures, a 0.02 inch thick sheet of the sametungsten-containing alloy was found to have a yield tensile strength of34,200 p.s.i., an ultimate tensile strength of 53,000 p.s.i., and anelongation of 42 percent.

Sheets of the above, fully recrystallized tungsten containing; alloyhaving dimensions of 5% inches, length, /2 inch width, and 0.010 to0.012 inch thickness were immersed in sulfuric acid at a temperature of200 C. (392 F.) for a total of 32 days.

The weight loss of the tantalum-tungsten alloy sheet was determined bycomparing the initial weight of the sheet with the weight of the driedsheet after the 32 days of immersion. This weight loss figure was thencalculated as a linear rate of corrosion of the alloy sheet by thesulfuric acid, expressed in inches per year. A similar sheet of puretantalum was immersed in the same bath of sulfuric acid at 200 C. (392F.) for 32 days, and the corrosion rate measured in terms of inches peryear.

The corrosion rate of the above tantalum-tungsten alloy was determinedto be 0.00115 inch per year, while the corrosion rate of pure tantalumwas determined to be 0.00224 inch per year.

The above test was repeated on another sheet of the abovetantalum-tungsten alloy having the above dimensions, while a similarsheet of an alloy containing weight percent of tantalum and 10 weightpercent of tungsten was also immersed in the same bath at a temperatureof 200 C. (392 F). At the end of 13 days the test was discontinued andthe corrosion rates calculated. The alloy of this invention containingabout 2.7 weight percent of tungsten had a corrosion rate of 0.00124inch per year while the alloy containing 10 weight percent of tungstenhad a corrosion rate of 0.00198 inch per year.

The superiority of the above alloy containing about 2.7 weight percentof tungsten was also determined by immersing strips of pure tantalum andthe above described alloy in 37 percent hydrochloric acid at C. (212 F.)for 24 hours. The corrosion rates calculated from this test were 0.0009inch per year for the tantalum-tungsten alloy containing about 2.7weight percent of tantalum, and 0.0016 inch per year for the puretantalum.

Thus it can be seen that the alloys of this invention have superiorcorrosion resistance over both pure tantalum and a tantalum alloy having10 weight percent of tungsten.

From the foregoing, it will be observed that numerous variations andmodifications may be effected without departing from the true spirit andscope of the novel concept of the invention. It is, of course, intendedto cover by the appended claims all such modifications as fall Withinthe scope of the claims.

What is claimed is:

1. A single phase, cold workable alloy havin high corrosion resistanceand consisting of from 2 to 3 weight percent of tungsten, from 0.05 to0.5 weight percent of columbium added as grain refiner, the balance ofsaid alloy being tantalum and minor impurities, and characterized by ayield tensile strength at ambient temperatures of at least 30,000 p.s.i.and an average corrosion rate expressed in terms of inches per year ofabout onehalf of the corrosion rate of unalloyed tantalum when exposedto concentrated sulfuric acid at a temperature of no more than about 200C. for a period of about 30 days.

2. The alloy of claim 1 consisitng essentially of about 2.7 weightpercent of tungsten and about 0.16 weight percent of columbium, thebalance of said alloy being essentially tantalum.

3. The alloy of claim 1 which has, at ambient temperatures, a yieldtensile strength at ambient temperatures of about 35,000 p.s.i., and anultimate tensile strength of about 53,000 p.s.i.

4. The alloy of claim 1 wherein the average corrosion rate is no morethan about 0.00124 inch per year.

5. The alloy of claim 1 wherein the average corrosion rate is betweenabout 0.00115 and about 0.00124 inch per year.

References Cited UNITED STATES PATENTS 2 081,820 5/1937 Kelley -174X3,136,635 6/1964 Field et a1. 75174 3,183,085 5/ 1965 France et a1 75174OTHER REFERENCES CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 14832

